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YOSEMITE NATIONAL PARK — Most visitors coming from foggy San Francisco welcome the intense late-July heat and sunshine here as they stroll or cycle through the lowlands and along the ambling Merced River. But to year-round inhabitants — both human (park employees) and nonhuman — the warmth is starting to wear out its welcome.
“Yosemite’s temperatures are on the extreme warm end of the spectrum,” said William Monahan, an ecologist who works for the National Park Service Inventory and Monitoring Division and co-wrote a report that analyzed climate variables (including temperature, precipitation, frost and cloud cover) in 289 national parks, dating back to 1901. “Temperatures over the last 10 to 30 years exceed 95 percent of the historic range of high temperatures it has experienced.”
The iconic California park is not alone. While precipitation patterns varied across the country, temperatures at roughly 80 percent of the national parks analyzed have been well above historic norms — not just for annual mean temperature, but also minimum temperature in the coldest month and mean temperature in the warmest quarter.
Monahan said that the study focuses on the past 10 to 30 years because that range “captures natural climate cycles” and is also long enough to account for the general time frames over which the National Park Service implements resource-management plans. His hope is that the report will “lay a foundation for quantifying and understanding the effects of climate change” and then identify ways to respond to the changes.
Looking for patterns
In addition to the straight-brimmed, government-issue straw Stetson, Sarah Stock wears many hats at Yosemite National Park. Against a backdrop of cackling stellar jays, with a doe and her suckling fawns in the near distance, she says, “I am the park ornithologist. I also oversee the mammal program. So, day to day, I am coordinating different bird and mammal program studies in the park. That ranges from songbird studies to bighorn sheep.”
As she oversees these studies and considers the resulting data, Stock tracks population density, species richness (the number of species in a given ecological community) and abundance of focal species. “I’m looking for changes in pattern … and distribution across a landscape, at different elevations, and then looking for changes and asking why those changes may be happening.”
She is detecting changes that are tied to two broad factors: increasingly severe, hot wildland fires and variations in snowpack depths.
Wildfires are a constant threat to Yosemite, especially amid the drought. Two current blazes erupted in late July. The 4,600-acre El Portal fire, just outside park boundaries, forced evacuations and destroyed at least two homes, while the Dark Hole fire threatened a grove of giant sequoias inside the park. But thus far these two are tiny compared with last summer’s Rim fire, which burned 77,000 acres within Yosemite (and 257,000 acres, total, over the course of three months, making it the third largest in California history). Rangers are now studying how the fire affected great-gray-owl habitat (the park constitutes the bird’s southernmost range and is home to two-thirds the population of this California state endangered species).
Great gray owls nest in tree snags, which is a tree that is dead or dying, but still standing. Forest fires often create snags, but the Rim fire burned so hot through the owl’s habitat that it incinerated most trees. “We are not detecting great gray owls at as many sites as before the fire,” Stock said, noting that 20 percent of the bird’s nesting habitat in the park fell within the fire’s footprint. It’s not just the loss of habitat that has eroded the owl’s habitat. Its main prey, the meadow vole, does not burrow and therefore could not escape the fire as it raged through that portion of the park, she explained.
“With predictions of more frequent, hotter fires, it means that great gray owls might not be able to come back to these areas under the present climate scenario,” Stock said.
Not all the changing patterns in the park are negative — or, at least, they do not all pose and immediate threat.
In partnership with the Institute for Bird Populations, Yosemite rangers have been conducting songbird studies since 1990. They involve capturing birds with mist nets, a fine nylon web used to safely capture and collect birds, and placing small bands on their legs to identify and track their activity thereafter. “Each one of our stations is at a different elevation gradient, from 4,000 to 8,000 feet, so every 1,000 feet we have a station,” said Stock. “A couple years ago, our collaborator looked at the data in relation to snowpack and found a correlation. In years with diminished snowpack, we are seeing higher productivity, which suggests a positive trend.”
Productivity is a measurement of young birds in relation to older birds — the more young birds you get the better, and the more "productive," the species. Should snowpacks continue to shrink, however, this near-term gain could turn into loss for the birds, due to a threshold after which low snowpack will begin to harm the populations of insects that the birds rely on for food, and the flora they need for shelter. “This is the third drought year in a row and if we continue to have dry years, we’re probably going to come right up against that threshold,” said Stock.
The challenges of responding to these types of changing patterns — not just in Yosemite but across all public lands — are myriad and daunting. One key is to find existing areas in the landscape, such as regions with consistent water resources and biodiversity, that offer benefits across species. Toward that end, Stock, Monahan and other scientists are studying ways to identify and promote “refugia,” which are naturally occurring landscape features that protect or promote not just one but a range of plant and animal species that might be endangered by rising temperature trends, water scarcity or some other stressor.
Land managers could take action to improve refugia for the benefit of target species, such as limiting non-native species to reduce competition for food. They will also study elevation and latitude, as new research shows that due to climate change and habitat loss, up to one-quarter of the total area of the U.S. National Park System is vulnerable to vegetation shifting up slope and northward.
Unfortunately, park managers lack a clear strategy to lessen harm from another change afoot in Yosemite: melting glaciers. Park geologist Greg Stock (Sarah’s husband) says the two remaining glaciers in the park are less than half the size they were back in 1883, the date of their earliest detailed records. One of the two glaciers, the Lyell, has been losing so much ice each year that it no longer moves downhill. The rapidity with which the Lyell is shrinking is important because the glacier contributes to the Tuolumne River headwaters during summer months, well after other sources in the headwaters grow dry.
“In dry years like this, there is very little water in the Tuolumne River except for at the edge of the glacier — it is a reliable source of water,” Greg Stock explained. It’s not just wildlife that relies on those headwaters during late summer months. The flow is also important to the many backpackers who trek into Yosemite’s famous Tuolumne Meadows.
“We’re attributing the melting glaciers primarily to warming summertime temperatures,” he said. Based on climate models, he thinks both of Yosemite’s remaining glaciers could disappear within 50 years.
Yosemite National Park is celebrating its 150th birthday this year. On its 200th birthday, the Half Dome and El Capitan peaks — features formed, Stock noted, by glaciers — will still make Yosemite a majestic, awe-inspiring valley. But the toll another half-century of extreme heat and diminishing snowpacks could take on the park’s ecosystems is unknown.
Both Sarah and Greg Stock field more climate-related questions from the visitors to the park today, compared with when they started working here eight years ago.
“Sometimes people say, ‘Well, yeah, the glaciers are retreating. So what? They grow and shrink with time naturally,’” said Greg Stock. “And that is true, of course. There are no longer 2,000 feet of [glacial] ice in Yosemite Valley. But those cycles of advance and retreat, in that past, had been driven by a certain set of things. We’ve added [greenhouse gas emissions, chiefly carbon dioxide] to that. There is not a big leap of logic to connect the higher CO2 in our atmosphere with the shrinking of these glaciers.”
Impacts such as owl-habitat loss or fluctuating songbird populations are likely hard for park visitors to internalize, but melting glaciers — even though they’re far in the backcountry and invisible to most visitors — is something visitors respond to viscerally.
Sarah Stock said talking to park visitors about climate is important, even when debates emerge. “Each person can have their own interpretation, but mine is based on what we’re seeing with data. The data are right,” she said with a smile.